PGBK not just for documents

Everything I have read has lead me to believe that the PGBK is used just for document. I have since found out that, that is not exactly true at least for some printers. I have an ix6820 that the for some reason the PGBK cartridge stopped working. But, I could still print great glossy pictures on it just fine. I could not print black and white documents, which was fine because I had the other printer that I could use. After a while I started having other issues with the ix6820 so I decided to get a new printer. I wanted one that used the same ink cartridges so I got an ip7220. When I got the new printer I just moved the cartridges from the old printer to the new printer.

Long story short the photos from the ip7220 where terrible. I tried everything I could think of the correct the color imbalance, but nothing worked. Then I thought to try one more thing. I figured the since the PGBK has been sitting in the ix6820 for years without being used that it was probably all dried out. So I replace that cartridge with the one that came with the new printer and lo and behold the photos came out great. So I have to conclude that the PGBK does effect the color on at least high quality glossy photos.